The present invention relates to a steel sheet such as hot-rolled steel sheet and cold-rolled steel sheet, and to a method for manufacturing the same.
Steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets are used in wide fields including automobiles, household electric appliances, and industrial machines. Since these steel sheets are subjected to some processing before use, they are requested to have various kinds of workability. For example, high strength hot-rolled steel sheets which are not subjected to deep drawing of 340 MPa or more strength are requested to have high stretch flanging performance during burring.
Recently, the request of users of steel sheets regarding the quality becomes severer than ever. The request includes not only further improvement in the workability but also the homogeneity in mechanical properties in a coiled product.
Responding to the requirements of the users, there are several proposals. For example, JP-B-61-15929 and JP-B-63-67524, (the term xe2x80x9cJP-Bxe2x80x9d referred herein signifies the xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d), disclose a method to improve the workability of high strength hot-rolled steel sheet by controlling the cooling speed after hot-rolled and by controlling the coiling temperature, and JP-A-9-241742, (the term: JP-Axe2x80x9d referred herein signifies the xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), discloses a method to improve the homogeneity of mechanical properties in a hot-rolled coil by continuation of the hot-rolling process.
The high strength hot-rolled steel sheets manufactured by the method disclosed in JP-B-61-15929 and JP-B-63-67524, however, failed to attain sufficiently superior stretch flanging performance. Also when the method disclosed in JP-A-9-241742 is applied to high strength steel sheet, homogeneous excellent mechanical properties cannot be attained.
Since the high strength hot-rolled steel sheets having texture consisting essentially of ferrite and martensite have superior balance of elongation and strength and give excellent workability, they are increasing in applications to various structural members and parts aiming at weight reduction of automobiles. Along with the ever-widening their application field, the use conditions have increased in their severity, so that further improvement in their workability is wanted. To increase the balance of elongation and strength of that kind of textured steels, further fine texture is required.
That type of textured steel is manufactured by cooling (primary cooling) from the state of Ar3 transformation point or above to the region of ferrite-austenite two phase temperatures, and by holding the temperature region for a specified time to enhance the ferrite transformation to enrich C to the austenite phase, then by rapid cooling (secondary cooling) to transform the austenite to martensite. Technologies to establish fine texture by specifying the manufacturing conditions are proposed. For example, JP-A-54-65118 discloses the technology to suppress the grain growth by regulating the primary cooling speed to 80xc2x0 C./sec or more. JP-A-56-33429 discloses the technology to obtain fine ferrite by applying the temperatures to start the primary cooling of from 720 to 850xc2x0 C. and by applying the primary cooling speeds of from 30 to 200xc2x0 C./sec. JP-A-60-121225 discloses the technology to obtain finely dispersed ferrite and to obtain fine martensite by applying cumulative drafts of 45% or more between the Ar3 transformation point and the (Ar3 transformation point +40xc2x0 C.).
However, all of JP-A-54-65118, JP-A-56-33429, and JP-A-60-121225 have limitation to establish fine texture because the technological investigation was conducted in a limited range of primary cooling speeds of 200xc2x0 C./sec or less assuming the application of cooling capacity of existing commercial facilities or experimental apparatuses.
It is an object of the present invention to provide a method for manufacturing steel sheets which have excellent workability including the stretch flanging performance and which have various strength levels with homogeneous mechanical properties.
To attain the object, the present invention provides a method for manufacturing steel sheet comprising the steps of: forming a sheet bar; forming a steel strip; primary-cooling; air-cooling; secondary-cooling; and coiling.
The step of forming the sheet bar comprises rough-rolling a continuously cast slab consisting containing 0.8% or less C by weight.
The step of forming the steel strip comprises finish-rolling the sheet bar at finish temperatures of (Ar3 transformation point xe2x88x9220xc2x0 C.) or more.
The step of primary-cooling comprises cooling the finish-rolled steel strip at cooling speeds of higher than 120xc2x0 C./sec down to temperatures of from 500 to 800xc2x0 C.
The step of air-cooling comprises air-cooling the primary-cooled steel strip during a period of from 1 to 30 seconds.
The step of secondary-cooling comprises cooling the steel strip at cooling speeds of 20xc2x0 C./sec or more after air-cooling.
The step of coiling comprises coiling the secondary-cooled steel strip at temperatures of 650xc2x0 C. or less.
For the case of continuously cast slab containing more than 0.8% C by weight, the step of forming the steel strip comprises finish-rolling at finishing temperatures of (Arcm transformation point xe2x88x9220xc2x0 C.) or more.
It is another object of the present invention to provide a method for manufacturing high strength steel sheet having excellent sheet shape and workability, which steel sheet has superior balance of elongation and strength by establishing fine structure without damaging the sheet shape.
To attain the object, the present invention provides a method for manufacturing steel sheet comprising the steps of: forming a slab; hot-rolling; primary-cooling; applying slow cooling or air-cooling; and coiling.
The step of forming the slab comprises the continuous casting of a steel consisting essentially of 0.04 to 0.2% C, 0.25 to 2% Si, 0.5 to 2.5% Mn, and 0.1% or less sol.Al, by weight.
The step of hot-rolling comprises rough-rolling the slab to prepare sheet bar, and finish-rolling the sheet bar. The finish-rolling is conducted by giving the reduction in thickness at the final stand of less than 30%, and is completed in a temperature range of from Ar3 transformation point to (Ar3 transformation point +60xc2x0 C.).
The step of primary cooling starts the cooling within 1.0 second after the completion of the hot-rolling, and the cooling speed is higher than 200xc2x0 C./sec down to the temperatures of from (Ar3 transformation point xe2x88x9230xc2x0 C.) to Ar1 transformation point.
The step of slow cooling or air-cooling is carried out at cooling speeds of 10xc2x0 C./sec or less for 2 seconds or more in the temperature range of from Ar3 transformation point to Ar1 transformation point.
The step of coiling is done after the secondary cooling at temperatures of 300xc2x0 C. or less.
It is another object of the present invention to provide a method for manufacturing high strength steel sheet having excellent workability such as local elongation.
To attain the object, the present invention provides a method for manufacturing steel sheet comprising the steps of: forming a sheet bar; finish-rolling; primary-cooling; applying slow cooling; secondary-cooling; and coiling.
The step of forming the sheet bar comprises rough-rolling a steel consisting essentially of 0.04 to 0.2% C, 0.25 to 2% Si, 0.5 to 2.5% Mn, and 0.1% or less Al, by weight.
The step of finish-rolling comprises finish-rolling the sheet bar at rolling temperatures of 1,050xc2x0 C. or less, cumulative reductions in thickness of 30% or more, and end temperatures of rolling of from Ar3 transformation point to (Ar3 transformation point +60xc2x0 C.).
The step of primary cooling comprises cooling within 1.0 second after completed the finish-rolling at cooling speeds of higher than 200xc2x0 C./sec through a cooling range where the difference between the temperature to start cooling and the end temperature of the cooling is in a range of from 100% to less than 250xc2x0 C.
The step of slow cooling comprises cooling of the primary-cooled steel sheet at cooling speeds of 10xc2x0 C./sec or less for a period of from 2 seconds to less than 20 seconds in a temperature range of from above 580xc2x0 C. to 720xc2x0 C.
The step of secondary cooling comprises cooling of the slowly cooled steel at cooling speeds of 30xc2x0 C./sec or more.
The step of coiling comprises coiling of the secondary-cooled steel sheet at temperatures of below 400xc2x0 C.
Furthermore, the present invention provides a method for manufacturing steel sheet comprising the steps of: forming a sheet bar; finish-rolling; primary-cooling; applying slow cooling; and coiling.
The step of forming sheet bar comprises rough-rolling a steel consisting essentially of 0.04 to 0.12% C, 0.25 to 2% Si, 0.5 to 2.5% Mn, 0.1% or less Al, by weight, and balance of substantially Fe and inevitable impurities.
The step of finish-rolling comprises finish-rolling the sheet bar at rolling end temperatures of Ar3 transformation point or above.
The step of primary cooling comprises cooling of the finish-rolled steel sheet within 1.0 second after completed the finish-rolling at cooling speeds of more than 200xc2x0 C./sec through a cooling range where the difference between the temperature to start cooling and the end temperature of the cooling is in a range of from 100xc2x0 C. to less than 250xc2x0 C.
The step of slow cooling comprises cooling the primary-cooled steel at cooling speeds of 10xc2x0 C./sec or less for a period of from 2 seconds to less than 20 seconds in a temperature range of from above 580xc2x0 C. to 720xc2x0 C.
The step of coiling comprises coiling the slowly cooled steel sheet at temperatures of from 400xc2x0 C. to below 540xc2x0 C.